In recent years, hybrid vehicles designed to obtain a driving force for the vehicle by a combination of a travel motor and an engine have been developed and have come into widespread practical use. Among hybrid vehicles under development and in widespread practice use is a vehicle which drives a generator by an engine to generate electricity, and charges a battery for supplying electricity to a travel motor (PHV), and a vehicle in which a battery can be charged even by an external commercial power supply (PHEV).
Known as such hybrid vehicles are those capable of switching among the following modes according to the operating status: an EV mode in which driving wheels are driven, with only a travel motor being used as a power source; a series mode in which a travel motor is used as a power source, and a generator is driven by an engine supply electric power to a battery and the travel motor; and a parallel mode in which an engine and a travel motor are both used as power sources.
The hybrid vehicle is configured such that the generator is driven by the operation of the engine and generated electric power is charged into the battery. For the driving of the generator, a target engine speed and a target engine torque are set, and the generator is driven, with the engine being controlled, so as to obtain desired generated power, with the result, that constant generated power is obtained (see JP-A-2003-9305). Thus, irrespective of the target charge power conformed to the status of the battery, the engine is operated at a constant engine speed, and a driver never feels a sense of incongruity due to changes in the engine speed.